1. Field of the Invention
The present invention relates to a wire harness designing method for designing a wire harness that is placed on a desired application subject and also to techniques related thereto, and more particularly concerns a wire harness flexure life estimating method and techniques related thereto which estimate the flexure life of a wire bundle up to disconnection due to repeated bending processes, the wire bundle being formed by binding wires, each having a conductor line coated with an insulating layer, and used for supplying electric signals and power from a power supply of a car, an industrial apparatus and an electric or electronic apparatus attached thereto.
2. Description of the Background Art
As conventionally known, many wires or wire bundles, each formed by binding a plurality of wires (in this specification, wires and wire bundles are generally referred to as “wire harnesses”), are used in cars and industrial apparatuses. Some of wire harnesses are placed at a position, such as a door portion and a seat portion, that is subjected to bending, and such wire harnesses tend to have disconnection after having received repeated bending deformations; therefore, the wire harness needs to be designed by taking its flexure life into consideration.
Conventionally, for example, wire harnesses of cars have been designed and examined while taking formation, etc. of cars into consideration on a car manufacturer side (hereinafter, referred to as “manufacturing station”), and these have been subjected to performance evaluation by using prototypes, and then manufactured.
In this case, it is difficult to completely take the flexure performance of a wire harness into consideration from the initial designing stage. Therefore, in the conventional method, after the stage in which the initial designing has been made, a prototype is formed, and in the event of any problem in evaluation tests on this prototype, designing revisions are carried out and the product is developed.
More specifically, FIG. 29 shows a designing sequence of a conventional wire harness.
First, as step T1, a designing plan is formed with respect to a vehicle body as a whole.
At the next step T2, a designing plan is formed with respect to the wire harness (indicated by “W/H” in FIG. 29) so as to match the vehicle body.
Then, at step T3, based upon the wire harness design thus planned at step T2, the wire harness is formed on a trial basis.
Successively, at step T4, the prototype wire harness is actually bent repeatedly so as to carry out flexure evaluation tests. Then, the results of the flexure evaluation tests are examined (step T5), and in the case when required flexure endurance is not obtained, a prototype is again formed at step T3, and the flexure evaluation tests (step T4) and the examinations of the results (step T5) are repeatedly carried out until the required flexure endurance has been obtained; thus, at the time when positive examination results have been finally obtained, the corresponding mass production is started (step T6).
In recent years, there have been strong demands for shortened developing periods and elimination of prototypes in the entire automobile field, and there also have been demands for improvements in the sequence of conventional jobs for manufacturing prototypes (step T3) and for executing flexure evaluation tests (steps T4, T5).
Moreover, in general, wire bundles, which are bridged over a door and the body of a car, are allowed to pass through a protective grommet used for the purpose of waterproof and prevention of scratches, and in this state, the grommet is secured to a hinge portion between the door and the body of the car. In this case, every time the door is opened and closed, the wire bundle is repeatedly extended and bent; therefore, it is important to estimate the flexure life of the wire bundle at this portion, in an attempt to manufacture a wire bundle and to select the product in the case of an attaching process.
Here, it is proposed that in order to estimate what degree of flexure life the wire bundle inside the grommet bridged over the door and the hinge portion has, a finite element method (matrix stress analyzing method) is adopted.
This finite element method is one of simulation techniques which analyze a stress distribution, etc. of a continuous body of a complex structural member by using a computer, and in this method, a structural member serving as an analysis subject is divided into finite number of elements by using triangular or rectangular finite element meshes, and a basic differential equation is set in each element, while a greater simultaneous linear equation (matrix equation) is solved so as to allow solutions of the respective elements to have continuity with solutions of the adjacent elements.
In this finite element method which divides the structural member into finite number of elements by using the finite element meshes and carries out analyses thereon as described above, in the case when the wire bundle is allowed to pass through the inside of the grommet as described above, since the grommet has a complex structure such as a bellow shape, considerably complex data need to be calculated and processed when the grommet is divided into a plurality of elements and subjected to a modeling process so as to apply the respective physical properties to each element. Moreover, the subject for estimation of the flexure life is a wire bundle that is a collection of a plurality of wire bundles; therefore, when each of the wire bundles is divided into individual elements, and subjected to a modeling process so as to estimate the flexure life of each element, a great amount of calculation processes are required.
As described above, when a wire bundle in a grommet is analyzed by using the finite element method, the load of calculation processes imposed on a computer becomes extremely high, resulting in a disadvantage of a long period of time required for calculations.
Moreover, the process for dividing the individual structural members such as the grommet and the respective wires of the wire bundle by using finite element meshes forms an extremely time-consuming process.
In general, at low temperatures including cold temperatures, an insulating layer (coating material) such as PVC becomes susceptible to cracks (coat cracks) due to fatigue fracture caused by repeated bending processes on the insulating layer. Consequently, since the conductor section (core line) at the portion having the crack is more susceptible to a local stress, disconnection at low temperatures is mainly controlled by the fatigue fracture in the insulating layer coating the conductor section.
Therefore, the applicant of the present invention has already proposed a method of estimating the flexure life of a wire (for example, in Japanese Patent Application No. 11-210650: hereinafter, referred to as “proposed example”) in which, with respect to cracks on the insulating layer of a wire harness at low temperatures, a master curve indicating the correlation between the amount of change in strain and the flexure life at the corresponding insulating layer portion is preliminarily obtained and the flexure life of a wire is estimated by using this master curve.
However, the above-mentioned proposed example is a method for finding the flexure life, under states where disconnection is mainly caused by cracks on an insulating layer, and the flexure life is found based upon the number of bending processes of the insulating layer up to disconnection; therefore, it is difficult for this method to estimate the flexure life under states where disconnection occurs in the conductor section (core line) with no cracks being generated on the insulating layer. For example, at normal temperature, in some cases, the conductor section might have a disconnection prior to the occurrence of a crack on the insulating layer. Moreover, in the case when a halogen-free resin material, or PE, etc., is used as an insulating layer, even at low temperatures, the conductor section might have a disconnection prior to the occurrence of a crack on the insulating layer. However, the proposed example fails to effectively estimate the flexure life in the case when the inner conductor section has a disconnection prior to the occurrence of a crack on the insulating layer at low temperatures.